1. Field of the Invention
This invention relates to an electrophotographic apparatus using a contact type charging device as a charging means, an image forming apparatus such as an electrostatic recording apparatus, and an image forming method using the contact type charging device as a charging means.
2. Description of the Related Art
Conventionally, a corona discharger which is a non-contact type charging device or the like has been used widely as a charger which charges an image carrier such as an electrophotographic photoconductor or the like. In recent years, the contact type charging devices have been used for the reasons of space saving, low cost, simplicity of power source, generation of less ozone and the like. As the contact type charging device, for example, a charging roller is well known, in which a conductive rubber layer is formed on a conductive core material and a protective layer is formed on the conductive rubber layer. This conductive rubber layer is put into contact with the image carrier under a predetermined pressure so as to charge the image carrier equally.
However, because the contact type charging device is directly in contact with the image carrier, the following problems arise depending on the materials of the conductive rubber layer and protective layer and production conditions thereof. That is, if a contact type charging device left for a long time under high temperatures and humidity in a fresh condition without contamination with toner is utilized to form images, a black stripe may occur corresponding to a contact position of the contact type charging device.
The reason why this problem arises is thought to be that because the contact type charging device is in firm contact with the image carrier at a predetermined pressure, low-molecular weight compounds inside the conductive rubber layer seep outside the protective layer and the surface layer is plastically deformed or the like, so that fitting between the image carrier and contact type charging device increases. When both the components are separated from each other, exchange of charges occurs therebetween. Further, if a charge having an opposite polarity to the charging potential polarity (hereinafter sometimes referred to as charging polarity) of the image carrier is loaded on the image carrier, the charge is injected/trapped (hereinafter sometimes referred to as electrostatic memory) in a bulk direction so that the charging characteristic or photo-sensitivity of the image carrier changes at positions in firm contact with the contact type charging device, thereby causing differences in density and half-tone images in particular.
Therefore, reduction of seepage of low-molecular weight compounds from the conductive rubber layer by appropriately selecting materials and the thickness of the protective layer or reduction of seepage of the low-molecular weight compound from the conductive rubber layer by appropriately selecting materials of the conductive rubber layer and the protective layer production conditions such as vulcanization conditions are worthy of consideration. However, in these cases, such inconveniences as charging failure are produced because of increased resistance in the contact type charging device, reduced freedom in selection of materials for providing the contact type charging device with its proper function, and reduced production efficiency because production process control becomes more difficult.
Further, although separation of the contact type charging device from the image carrier when not in the image forming cycle and separation of the image carrier from the contact type charging device using a spacer when not in use have been proposed, these proposals induce complicated structures thereby leading to increase of production costs.
The present invention has been proposed to solve the aforementioned problems of the prior art and it therefore is an object of the invention to provide an image forming method which, while maintaining freedom of selection of materials of the contact type charging device as a charging means, and selection of production processes, can prevent securely electrostatic memory to the image carrier due to exchange of charge caused when the contact type charging device is separated from the image carrier, so as to prevent securely an occurrence of black stripe or the like corresponding to a contact position of the contact type charging device, thereby enabling the formation of high quality images, and an image forming apparatus of a simple structure having the aforementioned functions.